# Selected Publications

### Direct observation of spin–orbit coupling in iron-based superconductors

*S. V. Borisenko, D. V. Evtushinsky, Z.-H. Liu, I. Morozov, R. Kappenberger, S. Wurmehl, B. Büchner, A. N. Yaresko, T. K. Kim, M. Hoesch, T. Wolf, N. D. Zhigadlo*

Spin–orbit coupling is a fundamental interaction in solids that can induce a broad range of unusual physical properties, from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors its role has, so far, not been considered of primary importance, with models based on spin- or orbital fluctuations pairing being used most widely. Using angle-resolved photoemission spectroscopy, we directly observe a sizeable spin–orbit splitting in all the main members of the iron-based superconductors. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is stronger than that of possible nematic ordering. The largest pairing gap is supported exactly by spin–orbit-coupling-induced Fermi surfaces, implying a direct relation between this interaction and the mechanism of high-temperature superconductivity.

### Experimental Realization of a Three-Dimensional Dirac Semimetal

*Sergey Borisenko, Quinn Gibson, Danil Evtushinsky, Volodymyr Zabolotnyy, Bernd Büchner, and Robert J. Cava*

*Phys. Rev. Lett 113, 027603 (2014) Editors' suggestion*

We report the direct observation of the three-dimensional (3D) Dirac semimetal phase in cadmium arsenide (Cd_{3}As_{2}) by means of angle-resolved photoemission spectroscopy. We identify two momentum regions where electronic states that strongly disperse in all directions form narrow conelike structures, and thus prove the existence of the long sought 3D Dirac points. This electronic structure naturally explains why Cd_{3}As_{2} has one of the highest known bulk electron mobilities. This realization of a 3D Dirac semimetal in Cd_{3}As_{2} not only opens a direct path to a wide spectrum of applications, but also offers a robust platform for engineering topologically nontrivial phases including Weyl semimetals and quantum spin Hall systems.

### Quasi One Dimensional Dirac Electrons on the Surface of Ru_{2}Sn_{3}

*Q. D. Gibson, D. Evtushinsky, A. N. Yaresko, V. B. Zabolotnyy, Mazhar N. Ali, M. K. Fuccillo, J. Van den Brink, B. Büchner, R. J. Cava & S. V. Borisenko*

*Scientific Reports 4, Article number: 5168 doi:10.1038/srep05168*

We present an ARPES study of the surface states of Ru_{2}Sn_{3}, a new type of a strong 3D topological insulator. In contrast to currently known 3D TIs, which display two-dimensional Dirac cones with linear isotropic dispersions crossing through one point in the surface Brillouin Zone, the surface states on Ru_{2}Sn_{3} are highly anisotropic, displaying an almost flat dispersion along certain high-symmetry directions. This results in quasi-one dimensional (1D) Dirac electronic states throughout the SBZ that we argue are inherited from features in the bulk electronic structure of Ru_{2}Sn_{3} where the bulk conduction bands are highly anisotropic. Unlike previous experimentally characterized TIs, the topological surface states of Ru_{2}Sn_{3} are the result of a d-p band inversion rather than an s-p band inversion. The observed surface states are the topological equivalent to a single 2D Dirac cone at the surface Brillouin zone.

Others about our work:

Iron-arsenic superconductors: order restored, Nature, Editor's summary

Condensed-matter physics: The pnictide code, Jan Zaanen, Nature 457, 546-547 (29 January 2009)

Hinweise auf die Ursache der Hochtemperatur-Supraleitung, Eberhard Karls, www.pro-physik.de

Arsenid-Supraleiter mit dem gewissen Etwas, Rainer Scharf, www.pro-physik.de

Superconducting in 3-D, Eric Hand, Nature News, doi:10.1038/news.2009.64

Grundlagenforschung: Ursache des Phänomens "Supraleitung" aufgeklärt

Auf dem Weg zur Supraleitung bei Raumtemperatur

Hochtemperatur-Supraleitung wird schrittweise enträtselt

Rätsel um Hochtemperatur- Supraleitung gelöst?

Elektronenspins als „Klebstoff